Among the classes of so-called "frequency independent" antennas are the equiangular antennas and the log-periodic antennas. Log-periodic antennas are so termed because any portion of the structure may be scaled so that the electrical properties repeat periodically with the logarithm of the frequency In principle, such antennas may be arranged to have any desired bandwidth, but in practice the bandwidth is limited by the manufacturing tolerances possible at the high frequency end, and the low frequency is ordinarily limited by the space required for the low-frequency antenna elements Frequency-independent and log-periodic antennas are well known in the art and are described, for example, in the text "Antenna Engineering Handbook" edited by Jasik, published by McGraw-Hill.
A particular type of log-periodic antenna is described in U.S. Pat. No. 3,210,767 issued Oct. 5, 1965 to Isbell. The Isbell antenna is a planar (all dipole elements lying substantially in one plane) log-periodic including a number of bays of half-wave dipoles fed by what amounts to an elongated balanced two-wire or two-conductor transmission line. The lengths of the dipole elements taper from a maximum at the low-frequency end to a minimum at the high-frequency or "feed" end.
Those skilled in the art know that antennas are reciprocal passive devices in which various properties are identical in both the transmitting and receiving modes. For example, the directivity and beamwidth are identical in both transmitting and receiving modes of operation. Ordinarily, description of antenna operation is couched in terms of either transmission or reception, the other operation being understood
When the feed transmission line of the Isbell antenna is fed with signal at a frequency near the center of the operating frequency band from the side of the transmission line having the relatively smaller dipole elements, the signal propagates along the transmission line. When propagating past the relatively small dipole elements near the feed point, the signal "sees" the dipole elements as relatively small capacitances which shunt the effective capacitance of the transmission line. The small radiating elements have relatively small radiation resistance in series with the relatively large reactance of the equivalent capacitance, and therefore radiate very little energy. Thus, the signal effectively propagates along the transmission line unaffected by the small dipole elements Eventually, the signal reaches regions in which the dipole elements coupled to the transmission line have lengths of approximately .lambda./4 (.lambda./2 for the entire dipole). In these regions, the propagating signal "sees" real dipole impedances or radiation resistances coupled across the impedance of the transmission line. The dipole impedances are of the same order of magnitude as the characteristic impedance of the transmission line. Consequently, at frequencies at which the dipole elements are approximately .lambda./2 long, energy is coupled from the transmission line to the elements and radiated thereby. The log-periodic dipole array is arranged so that more than one dipole receives significant energy at any midband operating frequency, so that an array of elements is formed for radiation at that frequency. The arraying of the elements and their relative phases results in radiation back toward the feed. Thus, a radiated beam is formed in the direction in which the array "points", viewing the array as a whole as an arrowhead pointing in a given direction. If energy were to propagate past the region in which the dipoles are about .lambda./2 long, it would encounter dipoles which approach lengths at which they individually produce multiple-lobed patterns and have impedances which couple energy from the transmission lines. However, most of the signal energy applied at the feed point is coupled out within the .lambda./2 dipole region, so little energy remains to flow to the relatively large dipoles, the radiation of which might perturb the desired antenna radiation pattern.
As so far described, the Isbell log-periodic dipole produces a singly polarized signal Antennas of the general type described by Isbell have been used for the horizontally polarized television receiving antennas, for broadband communication and the like. U.S. patent application Ser. No. 06/936,499 filed Dec. 1, 1986 in the name of Balcewicz, U.S. Pat. No. 5,023,619, describes the simultaneous use of two orthogonal linear polarizations for communication between widely spaced Earth stations. As mentioned in U.S. Pat. No. 4,590,480 issued May 20, 1986 in the name of Nikolayuk et al., singly-polarized or horizontally-polarized signals may not be optimum under all circumstances for television purposes. As mentioned therein, attention has been directed to the broadcasting of circularly polarized signals from a television transmitter in order to reduce the effects of ghosting and to provide uniformity of coverage. Orthogonally crossed log-periodic dipole arrays as described in the article "Space Antenna Selection and Design" by Brown et al., published in the October 1965 issue of Systems Design magazine, have long been known to be useful for simultaneous orthogonal linear polarization or, in conjunction with couplers for providing a quadrature phase shift, for transducing circularly polarized or elliptically polarized signals.
The crossed log-periodic dipole array antenna when fully deployed, as illustrated in the Brown et al. article, includes a transmission line arrangement or "boom" having an axis which lies parallel to the direction of electromagnetic propagation, and also includes two mutually orthogonal .lambda./2 dipole antennas at each of multiple bays. The dipole antennas at one end of the array have lengths of about .lambda./2 at the highest frequency of operation, and at the other end of the array have lengths of .lambda./2 at the low frequency of the operating frequency band Such an arrangement when in its deployed state may be difficult to mount in position. For example, for VHS television purposes in the United States, each of the two crossed dipoles at the low frequency end of the log-periodic array may be ten or more feet long, and when one of the dipoles is horizontal, the other is vertical. The dipole elements are large and for reliability must be relatively rigid. Such a structure is very awkward to store or manipulate. It is known to hinge each rigid dipole element near its juncture with the transmission line so that the elements fold to a stowed position parallel to the boom, in order to ease the storage problem. However, the problem of awkwardness in handling reappears once it is deployed ready for mounting. An automatic arrangement for deploying an antenna element is desirable, and especially one which is suitable for deploying the elements of a crossed log-periodic dipole array.
A deployable multibay crossed log-period antenna is described in U.S. Pat. No. 4,977,408, issued Dec. 11, 1990, in the name of Harper et al. The Harper et al. antenna includes a pair of crossed two-wire transmission-line feeds, and also includes a plurality of bays. Each bay includes four antenna elements, arranged in pairs as crossed dipoles. The antenna elements therein described are in the form of elongated flat spring-steel elements with a curved or "C-shaped" cross-section, similar to common steel tapes. Each bay includes a spool and a drum rotatable relative thereto. The four antenna elements of each bay are, in a stowed condition of the antenna, wound about the spool of the bay, with energy stored in the spring material The ends of the elements protrude through apertures in a drum surrounding the spool of the bay, and the elements are prevented from uncoiling from the stowed position by a locking apparatus which locks the drum to its associated spool. When the drum is released so as to be free to rotate, the energy of the coiled antenna elements rotates the drum, and the elements deploy by unwinding from their respective spools. The Harper et al. multibay antenna includes a plurality of such bays. The elements of each bay are wound about the spool of that bay in a direction opposite to that of adjacent bays, because of the need to make element connections to alternate poles of the feed transmission line from one bay to the next. Alternate bays unwind in opposite directions, which helps to reduce torques, which torques may be disadvantageous in a spacecraft application.
For spacecraft applications, reliability considerations make it imperative that testing be performed before launch. Thus, for operation near 50 MHz, (.lambda.=20 ft) a crossed log periodic dipole array might be required to deploy antenna elements as long as five feet. While two of these could deploy vertically, two others would be required to be horizontal. The prior-art "C" cross-section elements were found to be sufficiently stiff so long as the two "legs" of the C shape were in tension, but would buckle when in compression. In theory, proper orientation (a "U" orientation) would allow the antenna to extend to the full five feet. It was found, however, that the "C" antenna element tended to twist axially, and as soon as the twist approached 90.degree., buckling occurred. A flexible, deployable antenna element with improved stiffness is desired.